1. Field of the Invention
The invention relates to a positional detector device which is provided to detect license plates of motor vehicles.
2. Description of Prior Art
A motor vehicle control device has been introduced to administer entrances and exits of motor vehicles by reading the number of the license plate to obtain the numerical information on such places as parking lots and highway toll gates. In order to automatically recognize the numerical information of the license plate, it is necessary to detect a position of the license plate, and isolate the number to make the number distinct.
Whether or not the positional detection of the license plate is accurate significantly affects on the precision of the subsequent treatments. This is because it is particularly important to precisely carry out the positional detection of the license plate.
The following methods have been introduced to detect the position of the license plate from a frontal image of the motor vehicle.
(1) It is detected from straight lines (horizontal, vertical) of the contour of the license plate with the use of Huff's transformation. PA0 (2) It is detected by tracing horizontal and vertical edges as disclosed by e.g., Laid-open Japanese Patent Application No. 6-28596. PA0 (3) It is recognized by detecting the consecutive four or less digits of the number drawn on the license plate so as to distinct the number plate region with the use of two-value treatment and labeling treatment as disclosed by e.g., Laid-open Japanese Patent Application Nos. 60-193084 and 4-101286.
However, especially under adverse photographing conditions, it is difficult to detect the position of the license plate with the methods (1).about.(3) because an image is produced in which the contour of the license plate and its numerical region are illegible. The same holds true when the license plate has a similar coloration of the background components (e.g., bumper and chassis).
Therefore, it is a main object of the invention to provide a positional detector device for a license plate which is capable of detecting a positon of a license plate with high precision even when a photographed image is produced in which the contour of the license plate and its numerical region are illegible.